This invention relates to felts comprised predominately of poly(m-phenylene isophthalamide) staple fibers and a process for preparing the felts.
Snythetic fiber felts are known, e.g. from U.S. Pat. No. 2,910,763 which teaches preparation of the felts from batts of retractable staple fibers which have been needle-punched and then treated to cause the fibers to retract whereby the batt is compacted and the area of the batt is reduced, usually by at least 1/2. The product of the percentage of retractable fibers in the batt and the percentage retractability of the fibers must be not less than 2,000.
Retractable poly(m-phenylene isophthalamide) fibers are known, e.g. from U.S. Pat. No. 3,133,138 which describes a process for crystallizing amorphous retractable poly(m-phenylene isophthalamide) fibers to prevent excess shrinkage by heating drawn fibers at 300.degree.-350.degree. C while allowing no more than 10% shrinkage of the fibers. The crystallized poly(m-phenylene isophthalamide) fibers are only slightly retractable and have a high degree of crystallinity.
Commercial poly(m-phenylene isophthalamide) fibers are normally crystallized to avoid the excess shrinkage noted above. Also, shrinkage of amorphous poly(m-phenylene isophthalamide) fibers is cumulative, i.e. repeated heating cycles in hot water or steam followed by dry heat cause further shrinkage each cycle.
It is known to prepare fabrics and felts of crystalline poly(m-phenylene isophthalamide) fibers. These fabrics and felts are particularly useful in the filtration of hot gases, e.g. at 200.degree. C where other fibers such as polyester, acrylics, wool and nylon are not useful. Felts of crystalline poly(m-phenylene isophthalamide) fibers suffer from relatively poor dimensional stability and low strength. The lack of stability of these felts requires that the crystalline poly(m-phenylene isophthalamide) fiber batts be supported by a woven scrim to provide the required stability even though the poly(m-phenylene isophthalamide) fibers themselves have excellent dimensional stability. Even when supported by a scrim, crystalline poly(m-phenylene isophthalamide) fiber felts require calendering to achieve a sufficiently low air permeability. Unfortunately, such calendered felts are not completely stable in use, the air permeability exhibiting an undesirable gradual increase with length of time in service.